New TORCH_UCC_BLOCKING_WAIT env variable (#81791)
Cherry-pick of https://github.com/facebookresearch/torch_ucc/pull/95.
I recommend waiting until https://github.com/pytorch/pytorch/pull/81583 is merged first, so the CI is checking if this PR compiles correctly.
Marking this as a draft for now, will change to "ready for review" once https://github.com/pytorch/pytorch/pull/81583 merged.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81791
Approved by: https://github.com/kwen2501
